The present invention relates to the trimerization of ethylene in the presence of a tantalum-based catalyst to form 1 -hexene product at a selectivity of up to about 99 mole percent. Notable characteristics of the present invention are the high selectivity to 1-hexene product, an industrially important product for use as a feedstock for chemical synthesis and as a comonomer for olefin polymerization reactions, and the absence of a need for any chromium catalyst component in the reaction system.
The synthesis of 1-hexene from ethylene, an inexpensive and plentiful starting material, generally has been conducted by one of two synthetic routes. In the first route, a variety of metal-based catalytic processes have been reported in which ethylene is oligomerized to produce a relatively wide distribution of products, ranging from butene to much higher oligomers. Included in this distribution can be 1 -hexene. However, since several major products are synthesized, they must be separated to obtain pure products, such as 1-hexene. This purification increases production costs and does nothing to increase the selectivity of the product distribution.
In accordance with the second conventional route, several chromium-based catalyst systems have been reported in which ethylene is trimerized to 1-hexene with relatively high selectivity. The route is illustrated, for example, in U.S. Pat. No. 5,376,612 to Reagan et al, in U.S. Pat. No. 5,811,618 to Wu, in U.S. Pat. No. 5,750,816 to Araki et al, in U.S. Pat. No. 5,731,487 to Tamura et al and in U.S. Pat. No. 4,777,315 to Levine et al. A chromium-based catalyst route for trimerizing ethylene to 1-hexene is also illustrated in J. Catal. 1977 47 197 and J. Chem. Soc. Chem. Comm. 1989 674.
An object of the present invention is to provide an alternative process for synthesizing 1-hexene from ethylene with a high degree of selectivity.
Another object is to synthesize 1-hexene from ethylene with a high degree of selectivity by trimerizing ethylene in the presence of a catalyst system that does not require the presence of any chromium catalyst component.
Still another object is to provide a tantalum-based catalytic process for synthesizing 1-hexene from ethylene.
These and other objects and advantages of the invention will become apparent from the following detailed description.
In contrast with the known chromium-based catalytic processes for synthesizing 1-hexene from ethylene, the present invention provides a process for synthesizing 1-hexene with a high product selectivity by contacting ethylene with a catalytically effective amount of a tantalum-based catalyst composition comprising a tantalum compound, typically a tantalum halide, such as TaCl5 or Ta Br5, in combination with an alkylating component comprising a metal hydrocarbyl compound, a metal hydrocarbyl halide compound or an alkyl aluminoxane.
Typically the alkylating component would comprise a lower alkyl metal compound or a loweralkylaryl metal compound, wherein the metal would be selected from the group consisting of tin (Sn), zinc (Zn), aluminum (Al), lithium (Li) or magnesium (Mg). Non-limiting examples of alkylating components that are contemplated for use in this invention include, for example, tetramethyl tin (Sn(CH3)4), tetraethyl tin (Sn(CH2CH3)4), tetraphenyl tin, dimethyl zinc (Zn(CH3)2), methyl lithium (LiCH3), trimethyl aluminum (Al(CH3)3), allyl triphenyl tin (CH2xe2x95x90CHCH2Sn(C6H5)3), triethyl aluminum (Al(CH2CH3)3), n-butyl lithium (Li(CH3(CH2)3), dimethyl aluminum chloride ((CH3)2AlCl), methylaluminoxane (MAO), methyl magnesium bromide (CH3MgBr) or the like.
Generally, the tantalum compound and the alkylating component of the catalyst composition are combined in proportions to provide optimum selectivity to 1-hexene and optimum catalyst activity. Typically, a Ta/alkylating component ratio of from about 0.1 to 10 would be employed. Preferably, the ratio of Ta/alkylating component is from about 0.25 to about 2, and more preferably from about 0.5 to 1.
The synthesis reaction typically would be carried out in a slightly polar, non-coordinating solvent such as toluene, xylene, benzene, chlorobenzene, dichlorobenzene or the like. Aromatic solvents generally are preferred, although catalyst activity has been observed when using halogenated aliphatic solvents such as methylene dichloride, 1,1-dichloroethane, or the like.
The synthesis generally is conducted in batch reactions at pressures and temperatures selected to optimize reaction rates and selectivity. In general, a reaction pressure ranging from about 200 psig to about 1500 psig and a reaction temperature ranging from about 0xc2x0 C. to about 100xc2x0 C. would be employed. More typically, the reaction pressure would be on the order of from about 300 psig to about 1000 psig (e.g., 500-1000 psig), and the reaction temperature would be on the order of from about 25xc2x0 C. to about 70xc2x0 C. (e.g., 40xc2x0 C.-60xc2x0 C.).
When carried out in accordance with this invention, the ethylene trimerization process results in a reaction product containing primarily 1-hexene. Typically the reaction product would contain at least about 50 mole percent of 1-hexene, and more typically at least about 80 mole percent 1-hexene (e.g., from about 80 mole percent to about 99 mole percent). In preferred embodiments, the selectivity to 1-hexene is at least about 94 mole percent, and the reaction product would contain only minor amounts of by-products, such as C4, C8, C10 alkenes and higher oligomers. The amount of polyethylene in the reaction product, if any, typically would be negligible.
The invention will be more fully understood and appreciated when considered in light of the following illustrative examples, wherein all catalyst manipulations were carried out in a drybox under a nitrogen atmosphere and all reactions were carried out in a 125mL stainless steel, glass-lined reactor (autoclave) that was disposed in a heated oil bath and equipped with a magnetic stirring means. For each run of each example, the respective reactor was held at the indicated ethylene pressure with constant ethylene feed throughout each run. The actual trimerization reactions were carried out by allowing the reactors to reach the indicated reaction temperature with magnetic stirring for approximately 20 minutes. Then under a purge of ethylene, each reactor was charged with ethylene for the indicated period with magnetic stirring. For each run, the yield of 1-hexene was determined by gas chromatography using chlorobenzene as an internal standard.